prefer existing names for dependencies, over require$$x

[Rich-Harris]

This addresses #176 — common cases like this...

var x = require('x');

...will become like this...

import x from 'x';

...instead of this:

import require$$0 from 'x';
// ...
var x = require$$0;

It involves a bit of extra work, since we need to prevent this from happening in cases where x is reassigned (legally in CommonJS but illegally in ES modules), but I reckon it's worth it — the converted code is cleaner.

[StreetStrider]

Thanks for effort on such kind of a feature.